Many stripe-geometry semiconductor laser devices exhibit a lateral built-in guiding structure. Such guiding structures are usually fabricated by nonplanar growth over channels and terrace or mesa stripes via a multilayer liquid, vapor-phase or molecular beam epitaxy. Most of these prior art structures require a mask step to realign the ohmic contact stripe in the top epitaxial layer to confine the laser pumping current to the lateral built-in guiding structure itself. And to achieve superior device performance, accuracy of realignment is therefore critical. In the prior art, accuracy to within a micron is highly desirable; otherwise, device yield and performance are greatly degraded. Hence, the realignment process has evolved into a tedious process to achieve the required precision in alignment.
In the prior art two techniques that do not require this tedious realignment process in the fabrication of a multilayer epitaxy laser device are known, though not widely used. Furthermore, these two techniques themselves present additional problems and disadvantages.
One of the two has been reported by Shima et al. in "Buried Convex Waveguide Structure (GaAl)As Injection Lasers", (Appl. Phys. Lett. 38 (8), Apr. 15, 1981). In this report is described a self-aligned inner-stripe technique. It describes the requirement of a time-consuming epitaxial growth process both prior to and after etching or milling the nonplanar surface features into the substrate. Having both of these epitaxial growth processes represents an additional one to the usual growth process commonly used in fabricating multilayer epitaxy structures. Furthermore, the added epitaxial growth process requires relatively tight tolerances of growth thickness and thickness uniformity. And in the second technique, reported by Mori et al. in "Single Mode Laser with a V-shaped Active Layer Grown by Metalorganic Chemical Vapor Deposition: a V-shaped Double Heterostructure Laser" (J. Appl. Phys. 52(2), 5429 September 1981), a self-aligned V-guide technique is described. This technique requires an anomalous zinc diffusion process that is difficult to control and reproduce. The technique in accordance with the preferred embodiment of the invention, however, readily overcomes these prior art self-alignment problems as well as those inherent with the realignment techniques.
In accordance with the invention, an epitaxial growth process which allows grown epitaxial layers to retain, up to the topmost layer, some aspect of the nonplanar stripe-shaped surface features that are etched or milled into the substrate, is selected. An example of such an epitaxial growth process is that of organometallic vapor phase epitaxy (OMVPE), or metalorganic chemical vapor depositon (MOCVD). In this process, a proper combination of crystallographic substrate orientation, stripe orientation, cross-sectional shape and height of the surface feature in the substrate, for example, channel depth or terrace or mesa height, and epitaxial layer thickness of the multilayer structure is judiciously chosen. The top epitaxial layer is chosen to be a state-of-the-art, highly doped p-type (p.sup.+) contacting, or "cap", layer. Portions of this p.sup.+ cap layer on the planar surface of the multilayer structure are selectively removed in a free-etch process, that is, in a process using varying etch rates for differing crystallographic planes, or faces, in a semiconductor substrate. This novel application of a varying etch rate in the processing of a cap layer allows self-alignment and obviates the need of precise realignment etch masks. Specifically, when the selected portion of the p.sup.+ cap layer has been removed by etching, only the part of the p.sup.+ cap layer on the stripe-shaped surface remains. Metallization of the top layer then results in a nonblocking ohmic contact only to the remaining p.sup.+ cap layer stripes. This ensures the occurrence of selective pumping of the nonplanar guiding structure in the area located underneath these contacting off-planar surface stripes.